The Schiff base region of bacteriorhodopsin (BR), a light-driven proton pump, contains a pentagonal cluster, being composed of three water molecules and one oxygen each of Asp85 and Asp212. Asp85 and Asp212 are located at similar distances from the retinal Schiff base, whereas the Schiff base proton is transferred only to Asp85 during the pump function. The present FTIR study experimentally established the stretching vibration of water402 hydrating with Asp85 by use of various BR mutants, whose frequency (2171 cm-1 as the O-D stretch) indicates very strong hydrogen bond.